PCBs had often been used in the past as dielectric fluids in electrical equipment because they possessed a variety of very useful properties. However, their environmental persistence eventually resulted in a ban on such use. Because of the toxicity of the compounds, it has become necessary to devise techniques to eliminate or minimize the amounts of PCBs in the environment.
An illustrative chemical technique for reducing the level of PCBs present in organic solvents such as transformer oil is described in U.S. Pat. No. 4,663,027 for A. Mendiratta et al, and involves the addition of a combination of glycol and alkali metal hydroxide to form a reactive mixture. However, this type of direct chemical treatment is often not practical when the PCBs are located in areas such as landfill sites, river beds, and sewage sludge.
Another procedure for treating halogenated aromatics in organic waste is described in U.S. Pat. No. 4,477,570 issued to Colaruotolo et al. This patent teaches that microorganisms have been identified which have the capability of efficiently utilizing various aromatic organic chemicals as carbon sources for growth. Furthermore, it has been shown that some microorganisms are capable of growing in the presence of chlorinated aromatic compounds. However, PCBs which exist in weathered environmental soil often contain five or more chemically combined chlorine atoms per molecule, indicating that these types of compounds generally resist biodegradation.
In addition to the number of chlorine atoms per biphenyl nucleus, the location of chlorine atom substitution on the biphenyl nucleus is also an important factor influencing the resistance of PCBs to biodegradation. The positions at which chlorine may be attached to a biphenyl nucleus are shown below: ##STR1##
Reductive dechlorination of PCBs proceeds by stepwise removal of chlorines from the biphenyl nucleus. They are replaced by hydrogen atoms. When achieved by biological means, reductive dechlorination has practical value for effective clean-up with minimal ecological damage.
Dechlorination of PCBs in the environment by removal of meta and para chlorines has been reported for freshwater, estuarine, and marine sediments. This dechlorination has been attributed to anaerobic bacteria which exist in the sediments. However, the rate of dechlorination is usually slow, taking years or even decades.
The microbial population that resides in anaerobic sediments is very diverse. The microorganisms that are capable of dechlorinating PCBs may be only a small fraction of the total population. Since all of the microorganisms compete for the limited nutrients that are available, only those which have a biological advantage will actively grow. Such an advantage can result from the ability to use available nutrients more efficiently than competing organisms. An advantage can also result from an ability to utilize, for food or energy, compounds that other microorganisms cannot utilize in the same manner.
Certain PCBs, such as Aroclor 1260, which is a mixture of polychlorinated biphenyls comprised mainly of hexa- and heptachlorobiphenyls, are difficult substrates for microbial attack. In many environmental sites, only slight dechlorination of the PC Bs has occurred naturally. However, as reported by Bedard et al, in the Ninth Progress Report (1990) of General Electric Company's Research and Development Program for the destruction of PCBs, individual PCB congeners, such as 2,3,4,5,6-pentachlorobiphenyl, have been found to be capable of stimulating dechlorination of Aroclor 1260 in sediment by indigenous anaerobic microorganisms. Unfortunately, the addition of PCB congeners to a contaminated site is not an acceptable means of bioremediation because PCBs are regulated. Furthermore, in most instances, the PCB congener is not totally dehalogenated to biphenyl. Moreover the anaerobic microorganisms do not degrade the biphenyl nucleus which remains in the environment.
There is therefore a need in the art for an acceptable method of stimulating and accelerating microbial dechlorination of PCBs in aqueous sediments under anaerobic or low-oxygen conditions.